I think for engineers it's less of a question of "do I need chem spray," and more of a question of "can I get away with taking extinguisher instead?"
Off-hand, the weapons I look for to answer that question are flamer, banshee, and... uh...I think that's it? Oh yeah, Hades. And also gunners who are crazy enough to take incendiary rounds. I mean, hwacha can start fires, though it normally just explodes everything. Artemis can also start fires, but not very commonly. Flare gun always starts tons of fires, but extinguisher serves as a reactionary counter nearly as effectively as when chem spray is used protectively.
Weapons designed to actually start fires are just so popular, though, that at least one chem spray is almost always needed, and two is just way the hell safer.